Apparatus for Monitoring Particles in an Aerosol

ABSTRACT

The present invention relates to an apparatus ( 1 ) for monitoring particles in a channel ( 11 ) or a space comprising aerosol and to an ion trap arrangement in the apparatus. The apparatus ( 1 ) comprises an ejector ( 24 ), gas supply ( 6, 16, 18, 19 ) arranged to feed an essentially particle free ionized gas flow (C) to the ejector ( 24 ), a sample-inlet arrangement ( 2 ) arranged to provide a sample aerosol flow (A) from the channel ( 11 ) into the ejector ( 24 ) by means of suction provided by the gas supply ( 6, 16, 18, 19 ) and the ejector ( 24 ) for charging at least a fraction of the particles of the sample aerosol flow (A), and an ion trap ( 12 ) extending at least partly into ejector ( 24 ) for removing ions not attached to the particles. According to the invention the ion trap ( 12 ) is a provided as a metal trap wire.

FIELD OF THE INVENTION

The present invention relates to an apparatus for monitoring particlesand especially to an apparatus as defined in the preamble of independentclaim 1.

BACKGROUND OF THE INVENTION

Fine particles having diameter are formed in many industrial processesand combustion processes. For various reasons these fine particles aremeasured. The fine particle measurements may be conducted because oftheir potential health effects and also for monitoring operation ofindustrial processes and combustion processes, such as operation ofcombustion engines, especially diesel engines. Another reason formonitoring fine particles is the increasing use and production ofnanosized particles in industrial processes. The above reasons there isneed for reliable fine particle measurement equipments and methods.

One prior art method and apparatus for measuring fine particles isdescribed in document WO2009109688 A1. In this prior art method clean,essentially particle free, gas is supplied into the apparatus anddirected as a motive fluid flow via an inlet chamber to an ejectorprovided inside the apparatus. The clean gas is further ionized beforeand during supplying it into the inlet chamber. The ionized clean gasmay be preferably fed to the ejector at a sonic or close to sonic speed.The ionizing of the clean gas may be carried out for example using acorona charger. The inlet chamber is further provided with a sampleinlet arranged in fluid communication with a channel or a spacecomprising aerosol having fine particles. The motive fluid flow (i.e.the clean gas flow) causes suction to the sample inlet such that asample aerosol flow is formed from the duct or the space to the inletchamber. The sample aerosol flow is thus provided as a side flow to theejector. The ionized clean gas charges at least a fraction of theparticles. The charged particles may be further conducted back to theduct or space containing the aerosol. The fine particles of the aerosolsample are thus monitored by monitoring the electrical charge carried bythe electrically charged particles. Free ions are further removed usingan ion trap arranged downstream of the ejector. A typical ejector (orejector pump) includes a diverging cone (or diverging outlet diffuser)after the narrow throat to convert the kinetic energy of the gas topressure. This increases the the size and residence time of the gasinside the ejector. The residence-time increase slowers the timeresponse of the measurement apparatus based on the prior-art method(WO2009109688 A1).

One important demand for the fine particle monitoring apparatuses isreliable operation such that they may be operated long time periodswithout need for maintenance. In many applications, such as monitoringfine particles of combustion engines, it is also preferable that themonitoring apparatus may be operated continuously for conducting fineparticle measurements in real-time. Furthermore, in many cases the fineparticle monitoring apparatuses have to be installed in existing systemsin which there is only limited amount of space for the particlemeasurement apparatus. Usually industrial systems, combustion systems orother aerosol comprising systems are designed as compact as possible.Therefore, the fine particle measurement apparatus also has to be smallsized. The advantage of the small size is not, however, restricted onlyto the use of limited space. A more important advantage of the smallsize is the minimization of particle losses into the measurementapparatus. Further, small size enables faster time response of themeasurement due to the faster gas flow through the small sensing volume.

In many cases it's important that manufacturing costs of the apparatusis low. For this reason the structure of the apparatus should not be toocomplex to fabricate.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to provide an apparatus andarrangement so as to overcome the prior art disadvantages. The objectsof the present invention are achieved with an apparatus according to thecharacterizing portion of claim 1, which apparatus comprises an ion trap12 provided as a trap wire.

The preferred embodiments of the invention are disclosed in thedependent claims.

The present invention is based on the idea of providing an apparatus formonitoring particles of an aerosol by charging at least a fraction ofthe particles with an essentially particle free ionized gas fed into anejector and removing ions not attached to the particles substantiallydownstream of the ejector with an ion trap formed as wire or rod. Theion trap wire may be straight or it may be bended to a curvedconfiguration. The trap wire extends at least partly into the ejector.In one embodiment the trap wire is arranged to extend at least partlyinto the throat of the ejector. When the trap wire extends into thethroat (preferably the diverging part of it), the ion trap utilizes theejector structures and inner volume. As a result the total size of themeasuring apparatus and the sample-gas residence time in the sensor canbe reduced. The reduced residence time enables faster time response ofthe measurement apparatus.

Wire or rod-based ion traps are known as such from the prior art. USpatent application 2006/0284077 A1, TSI Incorporated, 21 Dec. 2006,describes an instrument for non-invasively measuring nanoparticleexposure includes a corona discharge element generating ions to effectunipolar diffusion charging of an aerosol, followed by an ion trap forremoving excess ions and a portion of the charged particles withelectrical mobilities above a threshold. Preferably the higherelectrical mobility elements are extracted using an electrostaticprecipitator with a tubular electrically conductive structure thatsurrounds a conductive element electrically isolated from the structure.The publication fails, however, to describe the advantages which areachievable by using an ejector and thus the instrument may suffer fromparticle losses.

The advantage of an ejector in a particle measurement apparatus which isbased on the measurement of electrical charge carried by particles isthat it allows a fast mixing of ions and particles. An ejector typicallyconsists of three parts: inlet nozzle, throat and diverging outletdiffuser. The inventor has found that mixing of the motive fluid flowand the side flow in an ejector is so efficient that in practice theparticles are charged by the ions carried by the motive fluid flowlatest in the throat of the ejector. Thus the ion trap used to removethe excess ions from the flow may be assembled at least partly into theejector and thus considerably shorten the flow path and minimize thesize of the particle monitoring apparatus. This also minimizes particlelosses into the particle measurement apparatus and enables faster timeresponse of the apparatus. Preferably, especially in the case ofcylindrically symmetric structure, the high-voltage electrode of the iontrap is rod-shaped, most preferably a wire, which does not essentiallyaffect the flow pattern inside the particle measurement apparatus. Theejector surfaces, especially the inner surface of the diverging diffuserand in some cases also the inner surface of the throat, work as the ioncollecting electrodes.

In a preferred embodiment of the present invention the ion trap isformed as a single wire or a rod providing both the ion trap wire andthe ion trap conductor. The ion trap is arranged to extend inside themeasurement housing from the inlet chamber to the ion trapping chamberand to the ejector in which it forms the ion trap.

The present invention has the advantage that the ion trap wire providesa simple mechanical structure for the ion trap. The simple mechanicalstructure enhances the reliable operation such that the apparatus formonitoring particles or the particle sensor may be operated long timeperiods without need for maintenance. The ion trap arrangement in whichthe trap conductor extends inside the measurement housing from the inletchamber to the ion trapping chamber and ejector provides also a compactstructure and decreases the external dimensions or diameter of theapparatus. The trap wire may also be formed into a configuration whichenables decreasing the dimensions of the ion trapping housing and thusthe external dimensions or the length of the apparatus. The trap wiremay also be formed into a configuration which enables use of moderateion trapping voltages.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be described in greater detail, inconnection with preferred embodiments, with reference to the attacheddrawings, in which

FIG. 1 is a schematic view of a prior part apparatus for monitoringparticles;

FIG. 2 is a schematic view of one embodiment of an apparatus formonitoring particles according to the present invention; and

FIG. 3 is a schematic view of one embodiment of an ion trap conductor.

DETAILED DESCRIPTION OF THE INVENTION

The FIG. 1 shows one embodiment of a prior art apparatus for monitoringparticles. The apparatus comprises an outer body 50 inside which isprovided a measurement housing 17 and at least part of the electricalcomponents and conductors 30 of the apparatus. As seen in FIG. 1 theelectrical components and connectors 30 are arranged substantiallyoutside of the measurement housing 17, thus between the outer body 50and the measurement housing 17.

The measurement housing 17 provides space in which the particlesmonitoring of the aerosol is conducted. A sample aerosol flow A isguided from a channel, duct or a space comprising aerosol inside themeasurement housing 17 for monitoring or measuring particles in theaerosol. The apparatus 1 is connected to an aerosol duct 11 in sidewhich is an aerosol flow F. Thus the apparatus 1 is arranged to monitorfine particles or particles in the aerosol flow F. The aerosol duct maybe exhaust duct of a combustion engine or the like. Alternativelyaerosol duct may be any space comprising aerosol or any duct or channelhaving an aerosol flow F. The space does have to comprise an aerosolflow, but the apparatus may also be arranged to monitor particles of asubstantially stationary aerosol for example particles of indoor air. Infigures the particle monitoring apparatus is connected outside of theaerosol duct 11 and to the side wall of the aerosol duct 11. Thisconfiguration needs openings to be made to the side walls of the duct11, but the apparatus does not significantly affect the flow conditionsinside the duct 11. In another embodiment the particle monitoringapparatus may be located inside the aerosol duct 11. In this embodimentthe apparatus may connected to the inner side walls of the duct 11 andno openings have to be made to the side walls of the duct. The apparatusmay be located inside an aerosol duct, or an exhaust duct, for examplewhen it is used to monitor particles of the exhaust gas of a combustionengine.

The apparatus 1 comprises a sample inlet 2 for guiding a sample aerosolflow A into the apparatus 1. The sample inlet 2 is in fluidcommunication with the aerosol duct 11 and inside of the measurementhousing 17 of the apparatus 1. The apparatus 1 preferably also comprisesa sample outlet 10 through which the analyzed sample aerosol flow B isexhausted from the measurement housing 17 and the apparatus 1. In theembodiment of FIG. 1 the analyzed sample aerosol B is returned to theaerosol duct 11. The sample outlet 10 may also be arranged to conductthe analyzed sample aerosol B directly to the ambient atmosphere or someother location, for example a container. Accordingly the apparatus 1does not necessarily collect or store the sample aerosol A received fromthe aerosol duct 11. In an alternative embodiment the apparatus may alsocomprise a sample-inlet arrangement 2 comprising one or more sampleinlets. Furthermore the apparatus may also comprise a sample outletarrangement 10 comprising one or more sample outlets.

The apparatus 1 comprises an inlet chamber 4 and the sample inlet 2 isarranged to provide a fluid communication between the aerosol duct 11and the inlet chamber 4. The apparatus further comprises a gas supplyfor supplying clean particle free gas C into the inlet chamber 4. Thegas supply comprises gas supply connection 18 via which the clean gasmay be brought from a gas source 19. The gas may be cleaned in a filteror the like for essentially removing particles from the gas such thatthe particle concentration in the pressurized gas is remarkably lowerthan the particle concentration in the sample aerosol flow A. The cleangas may be air or some other suitable gas. The clean gas may be alsofurther conditioned before feeding it into the inlet chamber 4. Theconditioning may comprise cooling or heating the gas and adjusting thegas flow speed and volume with a flow controller. The clean gas is thenfed to the measurement apparatus 1 through the gas supply connection 18.

The apparatus 1 further comprises a clean gas supply channel 16 throughwhich the clean gas is fed to inlet chamber 4 of the apparatus 1. Theclean gas supply channel 16 is in fluid communication with the gassupply channel 18 and comprises a nozzle head 6 opening into the inletchamber 4. The apparatus is further provided with an ionization device14 for ionizing at least a portion of the clean gas before or duringfeeding the clean gas from the nozzle head 6 into the inlet chamber 4.In the embodiment of FIG. 1 the ionization device 14 is arranged to theclean gas supply channel 16. In the embodiment of FIG. 1 the ionizationdevice is a corona needle 14 extending in the clean gas supply channel16. the ionization device 14 may also be another kind of electrodesuitable for ionization the clean gas. The nozzle head 6 and the coronaneedle 14 are advantageously arranged such that corona needle 14 extendsessentially to the vicinity of the nozzle head 6. This helps the coronaneedle 14 to stay clean and improves the ion production. The clean gasflow passing the corona needle 14 keeps the corona needle clean. Thecorona needle 14 is isolated from the clean gas flow channel walls andthe body 17 of the apparatus 1 by one or more electrical insulators 35.The walls of the gas supply channel 16 are preferably at the samepotential as the corona needle 14. According to the above mentioned thegas supply channel 16 is arranged to provide an essentially particlefree ionized gas flow C to the inlet chamber 4.

The apparatus is further provided with an ejector 24. The ejector 24comprises a converging-diverging nozzle 24 forming thus aconverging-diverging flow channel, the throat 8 of the ejector 24. Theejector 24 is a pump-like device utilizing the momentum of the main flowto create suction for a side fluid flow. The main fluid flow and theside fluid flow are at least partly mixed in the ejector 24. Afterpassing through the throat 8 of the ejector 24, the mixed fluid expandsand the velocity is reduced which results in converting kinetic energyback into pressure energy. In an alternative embodiment the apparatusmay also comprise one or more clean gas supply channels 16, coronaneedles 14 and ejectors 24.

In the embodiment of FIG. 1 the essentially particle free ionized gasflow C discharged from the nozzle 6 is fed to the throat 8 of theejector 24 as a main flow. Therefore the clean gas supply channel 16 andthe nozzle head 6 are arranged to feed the essentially particle free gasflow C at a high velocity into the throat 8. The velocity of theessentially particle free gas flow C is preferably sonic or close tosonic. In the ejector 24 the essentially particle free gas flow C formsa suction to the sample inlet 2 such that the sample aerosol flow A maybe sucked into the inlet chamber 4. The sample aerosol flow A forms aside flow of the ejector 24. The flow rate of the sample aerosol flow Ais depended essentially only on the geometry of ejector 24 and the flowrate of the essentially particle free ionized gas flow C. In a preferredembodiment the ratio of the main flow C to the side flow A is small,preferably less than 1:1 and more preferably less than 1:3. According tothe above mentioned there is no need for actively feed the sampleaerosol flow A into the apparatus 1, but it may be sucked by the bymeans of the clean gas supply and the ejector 24.

The essentially particle free ionized gas flow C and the sample aerosolflow are mixed in the inlet chamber 4 and in the ejector 24 such thatthe particles of the sample aerosol flow A are electrically chargedduring the mixing by the ionized clean gas flow C. The essentiallyparticle free ionized gas flow C and the sample aerosol flow A formtogether an ejector flow J discharging from the ejector 25 andspecifically from throat 8 of the ejector 24. The apparatus 1 furthercomprises ion trapping chamber 22 in side the measurement housing 17.The ion trapping chamber 22 comprises an ion trap 12 for removing ionsthat are not attached to the particles of the sample aerosol flow A. Theion trap 12 may be provided with a collection voltage for removing thementioned free ions. The voltage used for trapping free ions depends ondesign parameters of the apparatus 1, but typically the ion trap 12voltage is 10 v-30 kV. The ion trap 12 voltage may also be adjusted toremoved nuclei mode particles or even the smallest particles in theaccumulation mode. In an alternative embodiment the ion trap 12 isarranged to remove ions not attached to the particles from the ejectorflow J by an electric field, magnetic field, diffusion or a combinationthereof.

The sample aerosol and the essentially clean gas mixed together, theejector flow J, are discharged from the apparatus 1 through the outlet10 together with the charged particles of the sample aerosol. The outlet10 is provided in fluid communication with the ion trapping chamber 22for exhausting the discharge flow B out of the apparatus 1. The outlet10 may be arranged to supply the discharge flow B back to the aerosolduct 11 or to ambient atmosphere or some other location.

Particles of the aerosol F in the aerosol duct 11 are monitored bymeasuring the electrical charge carried by the electrically chargedparticles of the sample aerosol flow A. In a preferred embodiment theparticles of the aerosol F are monitored by measuring the electricalcharge escaping with the electrically charged particles from theapparatus 1 with the ejector flow J through the outlet 10. Themeasurement of the charge carried by the electrically charged particlesmay be measured by many alternative ways. In one embodiment the chargecarried by the electrically charged particles is measured by measuringthe net current escaping from the sample outlet 10. To be able tomeasure the small currents, typically at pA level, the whole apparatus 1is isolated from the surrounding systems. In FIG. 1 the apparatus 1 isprovided with installation insulators 13 for insulating the apparatus 1from the duct 11. An electrometer 34 may be assembled between theisolated apparatus 1 (i.e. a pint in the wall of body 50) and a groundpoint of the surrounding systems. With this kind of setup, theelectrometer 34 may measure the charge escaping from the isolatedapparatus 1 together with the ionized particles. In other words thiskind of setup measures the escaping current.

FIG. 1 shows one embodiment for monitoring the particles by measuringthe current escaping from the apparatus 1. The escaping current ismeasured with electrical arrangement 30. The electrical arrangement 30comprises a high voltage source 36 connected to the ionization device 14for providing a high voltage to the ionization device 14. The highvoltage source 36 is electrically isolated from the other system via anisolating transformer 38 and electrical insulators 35. The ionizationdevice 14 is in the same electrical potential as the walls of the gaschannel 16. The electrical arrangement 30 comprises further anelectrometer 34 assembled between the ionization device 7 and to a pointhaving a galvanic contact with the wall of the measurement housing 17.First contact of the high voltage source 36 is connected to theionization device 14 and the second contact is connected as a firstinput of the electrometer 34. The second input of the electrometer 34 isconnected to the wall of the measurement housing 17 and to the ion trap12. With this kind of electrical arrangement 30, the electrometer 34measures the charge escaping from the ion trapping chamber 22 and fromthe apparatus 1 with the ionized particles, e.g. measures the escapingcurrent.

The ion trap 12 prevents the free ions escaping from the apparatus 1.The ion trap 12 is connected to a collection voltage source 29 via anion trap conductor 25. In the prior art the ion trap 12 is provide asnet-like electrodes or plate like electrodes. FIG. 1 shows one prior artembodiment in which the ion trap 12 plate arrangement arrangeddownstream of the ejector 24 for removing free ions from the ejectorflow J. The ion trap 12 is connected to a collection voltage source 29via the ion trap conductor 25 arranged to extend from substantiallyoutside of the measurement housing 17 and specifically between the outerbody 50 and the measurement housing 17 of the apparatus 1. In analternative embodiment the ion trap may be arranged to remove ions notattached to the particles from the ejector flow J by an electric field,magnetic field, diffusion or a combination thereof.

FIG. 2 shows one embodiment of the present invention. In FIG. 2 the trapwire 12 is bended to extend at least partly into the ejector 24, whichconsists of the inlet nozzle 118, throat 8 and diverging diffuser 108.Specifically the trap wire 12 is arranged to extend at least partly intothe throat 8 of the ejector 24 or at least partly into a diffuser part108 of the ejector 24. This configuration of FIG. 2 enables the diffuser108 or the throat 8 of the ejector 24 to be used for removing free ionsnot attached to the particles. Therefore, the trap wire 12 utilizes thelength of the ejector 24 such that the length of the apparatus andspecifically the length of the ion trapping chamber 22 may be shortened.In tests it has been surprisingly noticed that the charging of theparticles is not affected when the trap wire 12 extends at least partlyinto the ejector 24 as that the ionization of the particles is carriedout well before the exit of the ejector 24.

As seen in FIG. 2, the apparatus 1 forming a particle sensor comprises ameasurement housing 17 inside which the ejector 24 is provided. Theinlet chamber 4 is arranged upstream of the ejector 24 and inside themeasurement housing 17. The inlet chamber 17 is provided with the gassupply 6, 16, 18, 19 feeding an essentially particle free ionized gasflow C to the ejector 24 for providing a sample aerosol flow A through asample-inlet arrangement 2 from a channel 11 or a space. The iontrapping chamber 22 is arranged downstream of the ejector 24 and insidethe measurement housing 17. The ion trapping chamber 22 is provided withan ion trap arrangement 12, 25, 26 comprising an ion trap 12 connectedto a collection voltage source 29 with a trap conductor 25 for providingthe ion trap 12 with a collection voltage for removing ions not attachedto the particles from an ejector flow J. The collection voltage source29 is located outside the measurement housing 17. As shown in FIG. 2,the trap conductor 25 is arranged to extend inside the measurementhousing 17 and specifically in side the measurement housing 17 from theinlet chamber 4 to the ion trapping chamber 22 for providing thecollection voltage to the ion trap 12. In one embodiment the trapconductor 25 is arranged to extend inside the measurement housing 17 andspecifically in side the measurement housing 17 through the inletchamber 4 and the ejector structure 24 to the ion trapping chamber 22for providing the collection voltage to the ion trap 12

The trap conductor 25 is electrically insulated from the measurementhousing 17. The trap conductor 25 may be provided with an outerinsulation layer for insulating purposes. The measurement housing 17 maybe provided with a trap conductor channel 28 through which the trapconductor 25 is passed inside the measurement housing 17 for separatingthe trap conductor (25) from the sample aerosol flow A, essentiallyparticle free ionized gas flow C and the ejector flow J. The trapconductor channel 28 may provide an electrical insulation for the trapconductor 25 inside the measurement housing 17. As shown in FIG. 2, thetrap conductor channel 28 may extend from the inlet chamber 4 to the iontrapping chamber 22 or through the inlet chamber 4 and the ejectorstructure 24 to the ion trapping chamber 22 such that the trap conductor25 may be passed inside the measurement housing 17 to the ion trappingchamber 22 such that the trap conductor 25 is not subjected to thesample aerosol A and to the essentially particle free ionized gas flowC. The ejector 24 may be formed from a ceramic material. The ceramicmaterial of the ejector 24 may be arranged to extend through he wholeinlet chamber 4 such that the trap conductor channel 28 may be providedto the ceramic material. The trap conductor channel 28 may thus extendthrough the ceramic material from the electronic arrangement through theinlet chamber and the ejector structure into the ion trapping chamber22. The trap conductor channel 28 may be formed by boring it through theceramic material. When ceramic material is used to form the ejector 24,the ceramic material may be coated with an electrically conductivelayer, such as a metal layer. The electrically conductive coating isfurther in connection with the walls of the measurement housing 17 suchthat the ceramic material is in the same potential with the measurementhousing 17. In an alternative embodiment the ceramic material may bereplaced with some other material. In a yet alternative embodiment thetrap conductor channel 28 may be provided with a separate channelelement arranged inside the measurement housing 17. The trap conductorchannel 28 prevents the trap conductor 25 from soiling due to the sampleaerosol flow A.

The trap conductor channel 28 may become contaminated as contaminants,such as fine particles of the sample aerosol, may enter the trapconductor channel 28 via the opening from which the ion trap 12 or anion trap holder 26 extends into the ion trapping chamber 22. The iontrap holder 26 may any mechanical and electrically conductive partarranged to support the ion trap 12 and to conduct the collectionvoltage from the ion trap conductor 25 to the ion trap 12. The ion trapholder 26 may be a separate part or it may be an integral part of theion trap conductor 25 or the ion trap 12 or both. To prevent soiling ofthe trap conductor channel 28 it may be provided with a sheath gas flowH flowing along the trap conductor channel 28 to the ion trap chamber 22between the inner walls of the trap conductor channel 28 and the trapconductor 25. The sheath gas flow H may comprise any suitable gas fromany suitable gas source. In a preferred embodiment the sheath gas flow His provided from the gas source 19 supplying also the essentiallyparticle free gas flow to the inlet chamber 4. The trap conductorchannel 28 and the ion trap conductor 25 have preferably differentcross-sectional shapes for provided a gap between the trap conductorchannel 28 and the ion trap conductor 25 in which gap the sheath flow Hflows. FIG. 3 shows one embodiment in which the trap conductor channel28 has a substantially round cross-section and the ion trap conductor 25a substantially rectangular cross-section.

The ion trap 12 may be a separate element electrically connected to thetrap conductor 25 with or without a trap holder 26. The separate iontrap 12 may be ion trap plate, net or some other substantially threedimensional ion trap arrangement.

In a preferred embodiment of the present invention the ion trap 12 andthe trap conductor 25 are provided as a single metal wire or a rodwithout a trap holder 26. The ion trap or the trap wire 12 may beimplemented as described earlier. Thus the trap wire 12 extends in thetrap conductor channel 28 as an ion trap conductor from which it isenters into the ion trapping chamber 22 as an ion trap 12. This providesa simple and compact solution for the ion trap arrangement. In this kindof ion trap arrangement the ion trap 12 may be longitudinal wire or arod arranged to extend at least partly into the ejector 24 or at leastpartly into the throat 8 of the ejector 24 inside the ion trappingchamber 22.

It is apparent to a person skilled in the art that as technologyadvanced, the basic idea of the invention can be implemented in variousways. The invention and its embodiments are therefore not restricted tothe above examples, but they may vary within the scope of the claims.

1.-10. (canceled)
 11. An apparatus for monitoring particles in a channelor a space comprising an aerosol, the apparatus comprising: an ejector;a gas supply that is arranged to feed an essentially particle-freeionized gas flow to the ejector; a sample-inlet arrangement that isarranged to provide a sample aerosol flow from the channel or the spaceinto the ejector by means of suction provided by the gas supply and theejector, and charges at least a fraction of the particles in the sampleaerosol flow; an ion trap that removes ions that are not attached to theparticles from an ejector flow that discharges from the ejector; andmeans for measuring the charge escaping from the ion trap; wherein theion trap is in the form of a trap wire, and extends at least partly intothe ejector.
 12. The apparatus according to claim 11, wherein the trapwire extends at least partly into the diverging diffuser of the ejector.13. The apparatus according to claim 11, wherein the trap wire extendsat least partly into the throat of the ejector.
 14. The apparatusaccording to claim 11, wherein the trap wire is arranged to remove ionsthat are not attached to the particles from the ejector flow by anelectric field, a magnetic field, diffusion, or a combination thereof.15. The apparatus according to claim 11, wherein the apparatus comprisesa measurement housing inside which is provided an inlet chamber that isarranged upstream of the ejector and in fluid communication with thechannel or the space through the sample-inlet arrangement; the apparatuscomprises an ion trapping chamber that is arranged downstream of theejector; and the trap wire is arranged to extend to the ion trappingchamber.
 16. The apparatus according to claim 15, wherein the trap wirecomprises a trap conductor that connects the trap wire to a collectionvoltage source for removing the ions not attached to the particles, andthe trap conductor is arranged to extend inside the measurement housingfrom the inlet chamber to the ion trapping chamber.
 17. The apparatusaccording to claim 16, wherein the trap conductor is arranged to extendin a trap conductor channel.
 18. The apparatus according to claim 15,wherein the trap wire comprises a trap conductor that connects the trapwire to a collection voltage source for removing the ions not attachedto the particles, and the trap conductor is arranged to extend insidethe measurement housing through the inlet chamber and the ejectorstructure to the ion trapping chamber.
 19. The apparatus according toclaim 18, wherein the trap conductor is arranged to extend in a trapconductor channel.
 20. The apparatus according to claim 17, wherein thetrap conductor channel is provided with a sheath gas flow that flows tothe ion trapping chamber between the inner walls of the trap conductorchannel and the trap conductor.
 21. The apparatus according to claim 11,wherein the trap wire and the trap conductor are provided as a singlemetal wire or a rod.